Heat treatable Al-Zn-Mg alloy for aerospace and automotive castings

ABSTRACT

A heat treatable aluminum alloy for shaped castings includes from about 3.5-5.5% Zn, from about 1-1.5% Mg, less than about 1% Si, less than about 0.30% Mn, and less than about 0.3% Fe and other incidental impurities.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application Ser. No. 60/564,813 filed on Apr. 22, 2004, which is fully incorporated herein by reference thereto. It is also closely related to the patent application “A Heat Treatable Al—Zn—Mg—Cu Alloy for Shaped Castings” filed concurrently with this application, and which is also incorporated herein by reference thereto.

FIELD OF THE INVENTION

This invention is an aluminum alloy for aerospace and automotive shaped castings, castings comprised of the alloy, and methods of making cast components of the alloy.

BACKGROUND OF THE INVENTION

Cast aluminum parts are used in structural applications in automobile suspensions to reduce weight. The most commonly used group of alloys, Al—Si₇—Mg, has well established strength limits. In order to obtain lighter weight parts, higher strength material is needed with established material properties for design. At present, cast materials made of A356.0, the most commonly used Al—Si₇—Mg alloy, can reliably guarantee ultimate tensile strength of 290 MPa (42,060 psi), and tensile yield strength of 220 MPa (31,908 psi) with elongations of 8% or greater.

A variety of alternate alloys exist and are registered that exhibit higher strength than the Al—Si₇—Mg alloys. However these exhibit problems in castability, corrosion potential or fluidity that are not readily overcome. The alternate alloys are therefore less suitable for use.

Where high strength is required, forged products are often used. These are usually more expensive than cast products. There exists the potential for considerable cost savings if cast products can be used to replace forged products with no loss of strength, elongation, corrosion resistance, fatigue strength, etc. This is true in both automotive and aerospace applications.

Casting alloys exhibiting higher tensile strength and fatigue resistance than the Al—Si₇—Mg material are desirable. Such improvements could be used to reduce weight in new parts or in existing parts which can be redesigned to use the improved material properties to great advantage.

INTRODUCTION TO THE INVENTION

The alloy of the present invention is an Al—Zn—Mg base alloy for low pressure permanent or semi-permanent mold, squeeze, high pressure die, pressure or gravity casting, lost foam, investment casting, V-mold, or sand mold casting with the following composition ranges (all in weight percent):

-   -   Zn: about 3.5-5.5%,     -   Mg: about 0.8-1.5%,     -   Si: less than about 1.0%,     -   Mn: less than about 0.30%,     -   Fe and other incidental impurities: less than about 0.30%.

Silicon up to about 1.0% may be employed to improve castability. Lower levels of silicon may be employed to increase strength. For some applications, manganese up to about 0.3% may be employed to improve castability. In other applications, manganese is to be avoided.

The alloy may also contain grain refiners such as titanium diboride, TiB₂ or titanium carbide, TiC and/or anti-recrystallization agents such as zirconium or scandium. If titanium diboride is employed as a grain refiner, the concentration of boron in the alloy may be in a range from 0.0025% to 0.05%. Likewise, if titanium carbide is employed as a grain refiner, the concentration of carbon in the alloy may be in the range from 0.0025% to 0.05%. Typical grain refiners are aluminum alloys containing TiC or TiB₂.

Zirconium, if used to prevent grain growth during solution heat treatment, is generally employed in a range below 0.2%. Scandium may also be used in a range below 0.3%.

The purpose of the present invention is to provide a range of aluminum alloys having good strength, good castability for forming shaped castings, good corrosion resistance and good thermal shock resistance. A fine grain size is often desirable for strength and for appearance, particularly for components which are anodized and then coated with a clear finish layer.

SUMMARY OF THE INVENTION

In one aspect, the present invention is an aluminum alloy including from about 3.5-5.5% Zn, from about 0.8-1.5% Mg. It contains less than about 1% Si, less than about 0.30% Mn; and less than 0.30% Fe and other incidental impurities.

In another aspect, the present invention is a heat treatable shaped casting of an aluminum alloy including from about 3.5-5.5% Zn, from about 0.8-1.5% Mg, and less than about 1% Si, less than about 0.30% Mn, and less than 0.30% Fe and other incidental impurities.

In another aspect, the present invention is a method of preparing a heat treatable aluminum alloy shaped casting. The method includes preparing a molten mass of an aluminum alloy including from about 3.5-5.5% Zn, from about 0.8-1.5% Mg, and less than about 1% Si, less than about 0.30% Mn, and less than 0.30% Fe and other incidental impurities. The method further includes casting at least a portion of the molten mass in a mold configured to produce the shaped casting, permitting the molten mass to solidify, and removing the shaped casting from the mold.

BRIEF DESCRIPTION OF THE DRAWING

The figure presents the results of ASTM G44 stress corrosion test on Al—Zn—Mg alloys with and without copper.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS AND COMPARISON WITH PRIOR ART ALLOYS

When referring to any numerical range of values herein, such ranges are understood to include each and every number and/or fraction between the stated range minimum and maximum. A range of about 3.5 to 5.5 wt % zinc, for example, would expressly include all intermediate values of about 3.6, 3.7, 3.8 and 3.9%, all the way up to and including 5.3, 5.35, 5.4, 5.475 and 5.499% Zn. The same applies to each other numerical property and/or elemental range set forth herein.

Alloys according to the present invention were tested in comparison with similar Al—Zn—Mg alloys containing copper. The samples were directionally solidified at a cooling rate of 0.1° C./sec. The results are presented in Table 1. TABLE 1 DS Casting T5 and T6 Properties T5 T6 Alloy Tensile Yield E % Tensile Yield E % Al—4.5Zn—1.2Mg 266 192 10 275 223.5 10 265.5 198 8 268 222.5 10 Al—4.5Zn—1.2Mg—0.4Si 238 174.5 10 309 230.5 16 238.5 173.5 10 312.5 233 16 Al—4.5Zn—1.2Mg—0.25Cu 285.5 207 10 269.5 210.5 10 287 205 12 276 210.5 14 Al—4.5Zn—1.2Mg—0.25Cu—0.12Zr 295 229 4 338.5 278.5 12 298.5 227.5 4 329.5 266 10

The first alloy shown in Table 1 was Al-4.5Zn-1.2Mg. Two samples were tested, each in T5 and T6 tempers. The tensile strength and yield strength are presented in megapascals, and the elongation in percent is presented, for two samples of the alloy, in both T5 and T6 tempers. This alloy is an example of the present invention.

The second alloy shown in Table 1 also has a composition in the range of the present invention. It contains Al-4.5Zn-1.2Mg-0.4Si. This shows lower values for tensile and yield strength than the previous alloy in T5 temper. However, it has significantly higher values for tensile strength, yield strength and elongation in T6 temper than did the previous alloy.

The third alloy shown in Table 1 is not within the composition range of the present invention. It is presented for comparison. The third alloy has higher values for tensile and yield strength and higher elongation values in T5 temper than the second alloy in T5 temper, but lower values for tensile and yield strength and lower value for elongation than the second alloy in T6 temper.

The fourth alloy shown in Table 1 is also not within the composition range of the present invention. It, also, is presented for comparison. The data presented illustrate the effect of zirconium, probably for preventing grain growth. The results for the T6 temper show very high values for tensile strength, yield strength and elongation.

Mechanical properties of shaped castings of an alloy according to the present invention were tested in a first plant trial, and the results are presented in Table 2. TABLE 2 First Plant Trial Al—3.5Zn—0.97Mg Temper Sample # Tensile Yield E % T5 1 219 175 14.6 2 211 169 8.6 As-cast 3 185 147 15.03 4 189 152 15.95

The composition for the first plant trial was Al-3.5Zn-0.97Mg. The table presents tensile strength and yield strength in megapascals, as well as elongation in percent. Two samples were tested in T5 temper, and two samples of the as-cast material were tested. It is noted that the elongation for the as-cast material had the extraordinary values of 15.03 and 15.95%.

Tests were also made in a second plant trial on an alloy containing slightly more magnesium than the alloy of Table 1. Data for the second plant trial are presented in Table 3 TABLE 3 Second Plant Trial Al—3.5 Zn—1.1Mg Tensile Yield E % Temper 210 161 16.7 160° C./1 hr 215 145 6.5 160° C./6 hrs 246 175 10.5 143° C./32 hrs

The data in Table 3 are for an alloy containing Al-3.5 Zn-1.1 Mg. This is an alloy according to the present invention. Data are presented for three different heat treatments. The first was 160° C. for 1 hour, the second was 160° C. for six hours and the third was 143° C. for 32 hours. The tensile strength and yield strength values in this table are expressed in megapascals, and the elongation is expressed in percent.

Table 4 presents data for the same alloy as the samples in Table 3. The samples reported in Table 4 were subjected to a T6 heat treatment that consisted of 471° C. for 3 hours, and then 527° C. for 10 hours followed by cold water quench. The samples were than aged as reported in Table 4, and the stress results in Table 4 were then obtained. The first line in the table is for a sample which was naturally aged only. TABLE 4 Al—3.5 Zn—1.1Mg after T6 Heat Treatment Ageing temp/time Tensile Yield E % Natural age only 274 MPa 138 MPa 24.5 160° C./6 hours 272 MPa 177 MPa 16.5 160° C./12 hours 287 MPa 201 MPa 16.0 160° C./18 hours 309 MPa 230 MPa 15.0 143° C./12 hours 270 MPa 175 MPa 17.5 143° C./32 hours 288 MPa 197 MPa 15.5 143° C./64 hours 311 MPa 239 MPa 11.5

Corrosion tests were also performed employing the ASTM G44 test which is the “Standard Practice for Exposure of Metals and Alloys by Alternate Immersion in Neutral 3.5% Sodium Chloride Solution”. In this test, stressed specimens are subjected to a 1-hour cycle which includes immersion in 3.5% NaCl solution for 10 minutes and then in lab air for 50 minutes. The samples were stressed at 75% of their yield strength, and the test was run for 180 days.

The figure shows the results of this test. It is seen that at high magnesium levels, copper is needed to prevent stress corrosion cracking. However, for the low magnesium levels of the present invention (about 1.2% Mg), copper is not required.

Presently preferred embodiments of the present invention having been presented above, it is to be understood that the invention may be otherwise embodied within the scope of the appended claims. 

1. A heat treatable aluminum alloy for shaped castings, said aluminum alloy comprising, in weight percent, alloying ingredients as follows: Zn: about 3.5-5.5%; Mg: about 0.8-1.5%; Si: less than about 1%; Mn: less than about 0.30%; and Fe and other incidental impurities: less than about 0.30%.
 2. An aluminum alloy according to claim 1 further comprising at least one grain refiner selected from the group consisting of boron, carbon and combinations thereof.
 3. An aluminum alloy according to claim 2, wherein said at least one grain refiner includes boron in a range from about 0.0025 to about 0.05%.
 4. An aluminum alloy according to claim 2, wherein said at least one grain refiner includes carbon in a range from about 0.0025 to about 0.05%.
 5. An aluminum alloy according to claim 1 further comprising at least one anti-recrystallization agent selected from the group consisting of zirconium, scandium and combinations thereof.
 6. An aluminum alloy according to claim 5 wherein said at least one anti-recrystallization agent includes zirconium in a range below 0.2%.
 7. An aluminum alloy according to claim 5 wherein said at least one anti-recrystallization agent includes scandium in a range below 0.3%.
 8. An aluminum alloy according to claim 1 wherein said zinc is at a concentration of about 4.2 to 4.8%.
 9. An aluminum alloy according to claim 1 wherein said zinc is at a concentration of about 4.4 to 4.6%.
 10. An aluminum alloy according to claim 1 wherein said magnesium is at a concentration of about 1.0 to 1.4%.
 11. An aluminum alloy according to claim 10 wherein said magnesium is at a concentration of about 1.1 to 1.3%.
 12. An aluminum alloy according to claim 1 wherein a concentration of iron in said alloy is less than about 0.3%.
 13. An aluminum alloy according to claim 1 wherein a concentration of manganese in said alloy is less than about 0.3%.
 14. A shaped casting of an aluminum alloy, wherein said alloy comprises alloying ingredients as follows: Zn: about 3.5-5.5%; Mg: about 0.8-1.5%; Si: less than about 1%; Mn: less than about 0.30%; and Fe and other incidental impurities: less than about 0.30%.
 15. A shaped casting according to claim 14 after T5 heat treatment.
 16. A shaped casting according to claim 14 after T6 heat treatment.
 17. A shaped casting according to claim 14 wherein said zinc is at a concentration of about 4.2 to 4.8%.
 18. A shaped casting according to claim 14 wherein said zinc is at a concentration of about 4.4 to 4.6%.
 19. A shaped casting according to claim 14 wherein said magnesium is at a concentration of about 1.0 to 1.4%.
 20. A shaped casting according to claim 14 wherein said magnesium is at a concentration of about 1.1 to 1.3%.
 21. A method of making an aluminum alloy shaped casting, said method comprising: preparing a molten mass of an aluminum alloy, said alloy comprising alloying ingredients as follows: Zn: about 3.5-5.5%; Mg: about 0.8-1.5%; Si: less than about 1%; Mn: less than about 0.30%; Fe and other incidental impurities: less than about 0.30%; casting at least a portion of said molten mass in a mold configured to produce said shaped casting; permitting said molten mass in said mold to solidify; and removing said shaped casting from said mold.
 22. A method according to claim 21 further comprising subjecting said shaped casting to a T5 heat treatment.
 23. A method according to claim 21 further comprising subjecting said shaped casting to a T6 heat treatment.
 24. A method according to claim 21 wherein said zinc is at a concentration of about 4.2 to 4.8%.
 25. A method according to claim 21 wherein said zinc is at a concentration of about 4.4-4.6%.
 26. A method according to claim 21 wherein said magnesium is at a concentration of about 1.0 to 1.4%.
 27. A method according to claim 21 wherein said magnesium is at a concentration of about 1.1 to 1.3%. 